The present invention relates to sonic emitting elements, to catheters incorporating the same and to methods of treating tissue in the body of a subject using such elements on catheters.
As described in International Publication WO 99/02096, the disclosure of which is hereby incorporated by reference herein, certain cardiac arrhythmias can be treated by ablating tissue in a ring around a pulmonary vein at the juncture between the pulmonary vein and the heart. As described in the ""096 publication, such ablation can be performed by threading a catheter having a thermal ablation element at its distal tip into the heart so that the tip is lodged within the appropriate pulmonary vein. The catheter may bear a balloon which is inflated within the vein and which holds the catheter in place. The ablating element is then actuated so as to apply heat in a region surrounding the ablating element. In certain embodiments taught in the ""096 publication, the ablating element includes a radio frequency (xe2x80x9cRFxe2x80x9d) emitting antenna.
Other embodiments described in the ""096 publication disclose the use of ultrasonic transducers. The ultrasonic transducer can be actuated to apply sonic energy through a fluid contained in the balloon, thereby heating the tissue surrounding the balloon. The preferred ultrasonic transducer illustrated in the ""096 publication is a ceramic piezoelectric element in the form of an elongated hollow tube having two cylindrical electrodes on its covering inner and outer surfaces. The entire piezoelectric element is excited by an alternating voltage and emits generally cylindrical sonic pressure waves.
Such a cylindrical transducer can apply only limited levels of power within a given volume of tissue. The ""096 publication suggests that a power level of 20 watts per centimeter of length along the vessel is adequate for ablation. While it is at least theoretically possible to heat tissues to a degree sufficient to cause ablation using relatively low power levels per unit of volume, such low-level heating prolongs the time required to heat the tissue. This, in turn, increases heat conduction from the heated tissues and increases the risk that adjacent tissue will suffer heat damage.
As also explained in the ""096 publication, such a cylindrical transducer typically must have an axial length at least equal to the wavelength of the sound in the transducer material. Accordingly, such a transducer tends to ablate tissue along a significant length of the pulmonary vein. This is undesirable because the desired treatment only requires ablation of a short ring of tissue surrounding the pulmonary vein to interrupt an undesired electrical conduction path. To avoid this undesirable result, the ""096 publication suggests masking the emissions from the ends of the cylindrical ultrasonic element. Such masking wastes the sonic power from the transducer, thus further aggravating the problem of providing adequate heating.
As a further alternative, the ""096 publication shows an ultrasonic emitter in the form of a hollow concave disk. The ""096 publication suggests that such an emitter can be physically rotated around the axis of a catheter so as to ablate a ring-like zone. This approach entails considerable practical difficulties inasmuch as entire rotatable assembly must be made to fit into a catheter which can be threaded through the circulatory system of the patient and into the pulmonary veins, typically a catheter having a diameter no more than a few mm.
Thus, despite all of the efforts devoted heretofore to development of procedures for ablating a circular region around a blood vessel, there are still substantial needs for further improvements.
The present invention addresses these needs. One aspect of the invention provides methods of ablating or otherwise treating tissues. A method according to this aspect of the invention desirably includes the step of threading a catheter into the circulatory system of the subject so that an emitting element carried on the catheter is positioned within a circulatory vessel with a central axis of the emitting element substantially aligned with an axis of the circulatory vessel. The emitting element is actuated to emit sonic energy in a pattern which is generally symmetrical about its central axis. The sonic energy is emitted so that the sonic waves from different portions of the emitting element intersect and mutually reinforce one another in a ring-like focal region surrounding the central axis, said focal region has an axial length less than the axial length of the emitting element. For example, sonic energy from different portions of the emitting element along the axial extent of emitting element may be directed at different angles to the central axis of the emitting element. Alternatively or additionally, sonic energy from different portions of the emitting element along the axial extent of emitting element may be emitted with different phases.
A further aspect of the present invention provides a sonic emitting element incorporating a piezoelectric element having a radiating surface substantially in the form of a surface of revolution generated by rotating a generatrix about a central axis. The term xe2x80x9cgeneratrixxe2x80x9d as used in this disclosure refers to a line or curve. The generatrix desirably includes a minimum point at a minimum distance from the central axis, and includes a first portion sloping away from the central axis in a first axial direction along the central axis from the minimum point. Desirably, the generatrix includes a second portion sloping away from the central axis in a second axial direction from the minimum point. The first and second portions of the generatrix desirably are curved and most preferably the first and second portions of the generatrix are segments of a circle. Thus, the radiating surface is in the form of an outwardly-facing concave portion of a toroidal surface.
The transducer may include one or more radially-expansible elements such as resilient spiral elements carrying a piezoelectric material such one or more layers of a polymeric piezoelectric film, together with electrodes used to actuate the film. Further aspects of the invention provide catheters incorporating sonic elements as discussed above.